1.Field of the Invention
The present invention relates generally to measurement-while-drilling systems and more particularly to a stabilization system on a drill string that improves the quality of formation evaluation measurements.
2. Background of the Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the "bottom hole assembly" or the "BHA") carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or "mud motor" which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors includes sensors for measuring the resistivity, dielectric constant, water saturation, porosity, permeability, density and other properties of the formation surrounding the wellbore. Such bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bid a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit.
One or more stabilizers are disposed on the drilling assembly to stabilize the drilling assembly against excessive vibrations of the drilling assembly caused by the cutting action of the drill bit. Stabilizers are usually pads that extend outward from the drilling assembly and come in contact with the borehole. Independently operated pads or ribs that can selectively apply force to the borehole inside have been proposed for altering the drilling direction, i.e., for geosteering the drill bid for directional drilling.
However, certain measurements relating to the formation evaluation parameters, such as nuclear magnetic resonance ("NMR") measurements, are very sensitive to any distortion of the magnetic field in the formation, which occur due to the movement of the magnets caused by normal vibrations of the BHA, even when commonly known multiple stabilizers are used. It is thus desirable to have a stabilization system that would automatically stabilize at least a section of the drill string containing sensors whose quality of measurement are susceptible to vibrations below a predetermined level during the drilling of the wellbore. The present invention addresses the above-noted need and provides a stabilization system for measurement-while-drilling sensors.